1. Field of the Invention
The present invention is directed to self-testing of an internal device in a closed system and, in particular, to an implantable medical device in a transcutaneous energy transfer (TET) system that performs automatic, periodic self-testing to verify its proper operation while minimizing power consumption.
2. Description of Related Art
In a variety of scientific, industrial, and medically related applications, it may be desirable to transfer energy or power (energy per unit time) across some type of boundary. For example, one or more devices that require power (e.g., electrical, mechanical, optical, and acoustic devices) may be located within the confines of a closed system, or “body,” in which it may be difficult and/or undesirable to also include a substantial and/or long term source of power. The closed system or body may be delimited by various types of physical boundaries, and the system internal to the boundary may be living or inanimate, may perform a variety of functions, and may have a variety of operational and physical requirements and/or constraints. In some cases, such requirements and constraints may make the implementation of a substantial and/or long term “internal” power source for internally located devices problematic.
In some closed systems, repeated entry into the system may be undesirable for a variety of reasons. In other closed systems, significant internal power requirements and a limited internal space may prohibit the implementation of a suitably sized internal power source. In yet other systems, contamination and/or security issues may pose particular challenges in implementing an internal power source. For any combination of the foregoing and other reasons, a power source external to the system and some feasible means of transferring power from the external source to one or more internal devices may be preferable in some applications.
One common example of a closed system is the human body. In some medically related and scientific applications, a variety of prosthetic and other medical-devices that require power may be surgically implanted within various portions of the body. Some examples of such devices include, but are not limited to, drug infusion pumps, pacemakers, defribllators, cochlear implants, sensors and stimulators. With respect to the human body, issues such as repeated reentry or surgery, internal space limitations, and contamination (e.g., infection) are factors to consider when selecting a suitable internal power source for some of these implantable medical devices.
Accordingly, in some medical implant applications, “transcutaneous energy transfer” (TET) devices are employed to transfer energy from outside the body to inside the body, to provide power to one or more implanted prostheses or devices from an external power source. One example of a conventional TET device is a transformer that includes a primary winding (or coil) external to the body and a secondary winding internal to the body. Both the primary and secondary windings generally are placed proximate to respective outer and inner layers of a patient's skin; hence, the term “transcutaneous” commonly refers to energy transfer “through the skin.”
Like any electronic device implantable medical devices are subject to possible malfunction or may cease functioning altogether. Because some of the medical devices provide life threatening functionality, it is imperative to test to ensure proper operation. Heretofore testing of implantable electronic devices was often initiated by an individual such as a physician, technician, nurse or patient upon engaging a button or key on an external control device in communication with the implantable medical device. Desirably such testing would be triggered periodically, for example, once every 24 hours, to verify proper operation. This disadvantageously would require someone to remember to manually activate or initiate the testing procedure.
In order to eliminate all possibility of human error in forgetting to initiate testing, means for automatically activating a self-testing sequence have been developed. For instance, U.S. Pat. No. 6,387,048 discloses an implantable sensor which includes electronic circuitry for automatically performing on a periodic basis, e.g., once every hour or once every day, specified integrity tests in order to verify proper operation of the sensor. A plurality of sensors are implanted in a patient in the same general area. Each sensor operates independently of the others. If all the sensors are functioning properly, then the output data obtained from each sensor should be approximately the same. The output data sensed by each sensor may thus be used as a cross-check against the output data sensed by the other sensors. However, the teaching of this patent is limited to checking of only high level output data detected by the sensor, and fails to check the low level operation of the components themselves. This is problematic in that the invention fails to identify the specific component that is subject to malfunction.
Other patents such as U.S. Pat. No. 6,740,075 disclose a TET system with self-testing functionality initiated at the external communication device. Software associated with the communication device, in turn, generates an Initiate Self-Test telemetry message that is transmitted via telemetry to the implantable device so that it may be tested as well. Another aspect of the testing functionality taught by the patented invention involves self-testing of the battery voltage of the implantable medical device. The communication device telemetry system sends messages to or receives messages from the medical device telemetry system, wherein the communication device is capable of performing a test of battery voltage with a load on the battery. Additional variations are described in which at least one of the following will occur, (1) the battery voltage is also automatically and periodically checked with the battery under a minimal load, (2) at least one selected electrical component is forced on to produce the load for testing, or (3) the test is made to occur at least in part when at least one selected electrical component is powered on in the performance of its normal operation, wherein the electrical component provides a load for the testing. Accordingly, this patented invention discloses self-testing of the implantable medical device (e.g., battery voltage) in response to a triggering signal generated by the external communication device.
Accordingly, the TET system taught by U.S. Pat. No. 6,740,075 requires a triggering signal from the external device to initiate automatic, periodic self-testing functionality of the internal battery power. In addition, except for battery power, all remaining components of the implantable medical device, in particular all other components whose malfunction could negatively impact the health of the patient, fail to be tested. Furthermore, such testing of the battery voltage is conducted periodically based solely on the expiration of predetermined periodic time periods or each time the device is powered on and thus is inefficient from an energy consumption perspective. As discussed above, in a closed system such as that employing an implantable medical device and external control unit each has its own coil for receiving/transmitting radio frequency signals therebetween. In addition, each of the implantable medical device and external control unit has its own associated power source, e.g., a battery, for powering its associated circuitry and its associated components. The battery, regardless of whether primary/non-rechargeable or secondary/rechargeable, has a limited lifespan and a predetermined amount of energy or power before having to be replaced or recharged. Testing to verify that the implantable medical device is working properly consumes energy from the limited internal battery power source thereby reducing its overall lifespan. Accordingly, heretofore the advantages of automatic, periodic self-testing of an implantable medical device to verify proper operation had to be weighed against the disadvantageous consumption of battery power and thus reduction in lifespan.
It is therefore desirable to develop an improved TET device that solves the aforementioned problems by conducting automatic, periodic self-testing functionality of multiple components, preferably all components whose malfunction could negatively impact the health of the patient, of the implantable medical device without triggering from an external device. Furthermore, it would be beneficial to design an improved TET device that automatically initiates periodic self-testing of the implantable device, of any number of one or more components, while minimizing or optimizing the amount of energy consumed or drawn from its internal power source.